In order to be able to respect the increasingly restrictive limits of emission imposed by recent anti-pollution standards, also in internal-combustion engines with small displacement (even just 50 cc) for motor vehicles it is necessary to use electronic-injection fuel supply instead of traditional supply to carburettors.
In an electronic-injection fuel-supply system for an internal-combustion engine with small displacement, an electrically actuated fuel pump draws the fuel from a tank at atmospheric pressure and supplies it to the injector. It is necessary for the fuel pump to have a very low electric-power absorption compatible with the electric power generated by the electric generator when the internal-combustion engine is idling.
The amount of fuel that is injected by an injector is a function both of the injection time (i.e., of the interval of time in which the injector is kept open) and of the fuel-supply pressure. Consequently, when the electronic-injection fuel supply is used, it is necessary to guarantee that the fuel-supply pressure is constant and equal to a predetermined design value.
In known internal-combustion engines with small displacement, a high-efficiency fuel pump is used (to keep the electric-power absorption low) with constant flow rate of fuel associated to a pressure regulator, which keeps the fuel-supply pressure constant and equal to the predetermined design value. Consequently, the fuel pump supplies to the injector a flow rate of fuel that is always constant irrespective of the engine r.p.m., and the pressure regulator recycles the excess fuel to the tank to keep the fuel-supply pressure constant and equal to the predetermined design value.
In other words, the fuel pump is sized to supply in each condition of operation an amount of fuel exceeding the effective consumption, and provided downstream of the fuel pump is the pressure regulator, which keeps the value of the fuel-supply pressure constant and equal to the predetermined design value, discharging the excess fuel towards a recalculation channel that sends the excess fuel back into the tank. In this case, the fuel pump must be sized to supply an amount of fuel equal to the maximum consumption possible. However, said condition of maximum consumption possible occurs rather seldom, and in all the remaining conditions of operation the amount of fuel supplied by the fuel pump is much greater than the actual consumption, and hence a considerable portion of said fuel must be discharged by the pressure regulator into the tank.
It is evident that the work performed by the fuel pump to pump the fuel that is subsequently discharged by the pressure regulator is “useless” work. Consequently, the electronic-injection fuel-supply system has as a whole a very low energetic efficiency. Furthermore, the pressure regulator and the recirculation channel connected to the pressure regulator are rather cumbersome and increase the overall costs of the electronic-injection fuel-supply system.
In an internal-combustion engine with small displacement, the high consumption of electrical energy is particularly burdensome during idling, in so far as during idling the electric-current generator of the engine has a modest capacity of generation. Consequently, during idling the operation of the fuel pump may be irregular owing to lack of an adequate electric power, and hence also the fuel injection and combustion may be irregular.
EP1306544A1 discloses an electronically controlled fuel injection device constructed from a plunger pump, a circulation passage which circulates fuel that has been pressurized in the initial region of the pressure-feeding stroke, a valve body which blocks the circulation passage in the later region of the pressure-feeding stroke, an inlet orifice nozzle which allows the passage of fuel whose pressure has been increased in the later region of the pressure-feeding stroke, an outlet orifice nozzle which is used to circulate some of the fuel that has passed through the inlet orifice nozzle back into the fuel tank, an injection nozzle which injects an amount of fuel equal to the difference between the fuel that has passed through the inlet orifice nozzle and the fuel that has passed through the outlet orifice nozzle, and control means for controlling the plunger pump in response to the cycle of the engine.